Pre-heating and pre-reduction of a metal oxide

ABSTRACT

The specification discloses a process for pre-heating and pre-reducing metal oxide ores. The process comprises introducing particles of an oxide ore entrained in a gas through a port into a treatment chamber. Inside the treatment chamber teh stream of entrained particles combines with a stream of high temperature reducing gas in such a way that the particles are heated rapidly and enter into flow patterns whereby contact with other particles and the internal surface of the treatment chamber is minimized. The stream of entrained particles and the stream of high temperature reducing gas are substantially co-current. A treatment chamber elongated in the direction of co-current flow is described in the specification. The hot off-gases may be derived from a molten bath reactor and comprise a high concentration of carbon monoxide and hydrogen.

FIELD OF INVENTION

The present invention relates to a process for pre-heating andpre-reducing a metal oxide.

BACKGROUND OF THE INVENTION

Off-gases from smelt-reduction furnaces are of high temperature and maycontain significant quantities of reducing gases such as carbon monoxideand hydrogen. It would clearly be of economic advantage to recover atleast some of the sensible heat and make use of some of the reducingpotential of the gases.

A number of prior art processes are known which provide eitherpre-heating or pre-reduction of metal oxide ores. Two processes, one bythe Kawasaki Steel company and the other by the Nippon Kokan company,combine both features.

Generally speaking, the known processes entail one or other of thefollowing disadvantages, and sometimes more than one:

several treatment steps may be required;

expensive coke or other reductants may be required; or

there may be temperature limitations leading to low pre-reduction, highresidence time or low pre-heat temperature achieved.

If the temperature to which the oxide ore is exposed is too high, theparticles may soften, leading to accretion on the apparatus and/oragglomeration of particles.

In the case of chromite ore, the pre-heating and/or pre-reductionprocesses described in the prior art are generally speaking limited totemperatures of around 1200° C., at which temperature reduction is veryslow.

The specification of U.S. Pat. No. 4,566,904 discloses a process forusing exhaust gases from a melting crucible to pre-reduce iron ore. Theexhaust gases are first reduced and cooled with a reductant such asnatural gas. The cooled reducing gas is then used to pre-reduce the ironore in a shaft furnace, a circulating fluidized layer or a fluidizedbed. The optimum temperature of the cooled reducing gas if 900° C. whichis considerably less than the temperature of the exhaust gas.Consequently this process involves a considerable loss of sensible heatwhich is used to increase the concentration of reductants in the exhaustgas.

The specification of U.S. Pat. No. 4,629,506 describes the production offerrochromium from a ferriforous chrome ore. The ore is heated in arotary kiln for 20 minutes to 2 hours to a temperature in the range from1480° C. to 1580° C. An atmosphere containing carbon monoxide ismaintained inside the kiln to reduce the ore. The resultant plastic massis cooled, crushed and magnetically separated into a coal rich fractionand a metal rich fraction. Preferably, the metal rich fraction isseparated by dry density separation into a metal poor slag fraction anda metal rich alloy fraction, the slag rich fraction is crushed and ametal rich slag fraction is extracted by magnetic separation. The metalrich slag fraction is added to the metal rich fraction and both are thenmelted in a crucible for further processing.

The process described in U.S. Pat. No. 4,629,506 involves a number ofprocess steps thereby resulting in additional capital and operatingexpenditure when compared with a process requiring less steps.Furthermore, the exhaust gases from the crucible are preferably used ascarrier gases for blowing coal and ore into the crucible or for lowtemperature coking of the coal.

U.S. Pat. No. 4,851,040 describes a process for producing iron from finegrained iron ore by direct reduction. The process involves insertingsponge iron and coal fines or a low temperature carbonized coal into aniron bath and injecting oxygen to produce a reducing gas and iron. Thereducing gas is used to reduce a pre-heated carbon coated fine grainediron ore in a "fluidized bed" at a temperature in the range from 700° C.to 1100° C. Spent reducing gas is used to preheat the fine grained oreto a temperature in the range from 450° C. to 700° C. as well as coatthe grains of ore with fine grains of carbon. The fine grains of carbonare deposited on the grains of ore by decomposition of carbon monoxide.The carbon layer on the grains of ore prevents the grains from stickingduring the reduction phase.

A process developed by Kawasaki Steel KK uses fine, unagglomerated ore;see JP 59080706. Pre-heating and pre-reduction is performed in afluidized bed. Heat and carbon monoxide-rich reducing gas are suppliedfrom the smelting reduction furnace off-gas and are supplemented by theinjection of hydrocarbon gas. The pre-mixing of the furnace gas at1350°-1400° C. with cooler hydrocarbon gas, for example, methane orpropane, results in a cooler gas mixture such that the bed temperatureis about 1200° C. It is believed that, at this temperature, a residencetime of 12-15 hours is required for substantial reduction of SouthAfrican chromite with a mean particle diameter of 325 μm. It is alsobelieved that only limited reduction of the iron and chromium oxides wasachieved when furnace gas, comprising carbon monoxide, was used alone.The hydrocarbon gas makes major contribution to the reduction of thechromite.

It is believed that the disadvantages of this process are the need foraddition of hydrocarbon gas to achieve substantial reduction, and thelow temperature of the fluidised bed, which causes the low rates ofreaction but is necessary to prevent softening of the chromite feed andsubsequent agglomeration of the particles within the fluidised bed. Itseems that the consequent high residence time prevents all chromite orefeed from being pre-heated and pre-reduced. (Some chromite ore isinjected directly to the smelting reduction furnace.

An object of the present invention is the provision of a process forpartly or almost completely reducing fine to coarse-grained metallicoxides, in particular metal ores, whereby the metallic oxide particlesdo not agglomerate in any appreciable amount but exist after theprereduction in the form of a granular, pneumatically conveyablematerial, and these partly reduced particles can be supplied to a finalreducing process, preferably a smelting reduction process, withoutrequiring any further elaborate processing steps.

Accordingly the present invention provides a process for pre-heating andpre-reducing a metal oxide which process comprises forming a stream ofmetal oxide particles and hot reducing gas to heat and to reduce atleast partially the metal oxide particles wherein the hot reducing gashas a temperature in excess of that at which the metal oxide particles,particles contained in the stream of reducing gas or both exhibit stickycharacteristics.

Metal oxide particles exhibit sticky characteristics when heated to atemperature range in which one or more phases present in the oxide existas liquids and the remaining phases continue to exist in the solidstate. This normally occurs first at a eutectic or peritectic point inthe multi-component system. When all the phases present exist in theliquid form, the metal oxide particles no longer exhibit stickycharacteristics. The temperature at which metal oxide particles begin toexhibit sticky characteristics and the range of temperatures over whichthey exhibit sticky characteristics varies from one metal oxide toanother and one mineral mixture to another. On the low side are systemssuch as Al₂ O₃ -FeO-SiO₂ and FeO-Fe₂ O₃ -SiO₂ where certain compositionsare liquid at 1150° C. On the high side are systems such as CaO-MgO-SiO₂in cases where the bulk of iron or other reducibles have been extractedfrom fluxes and gangue minerals. Such systems have melting points around1300° to 1350° C. In general stickiness associated with metallization ofiron can occur at temperatures down to 600° C. but is most rapid attemperatures in excess of 1000° C. This threshold is increased by100°-200° C. when heating ores containing manganese or chromium.

The hot reducing gas may be a synthesis gas derived directly from thecombustion of natural gas or coal in the presence of steam. Preferably,however, the hot reducing gas is an off-gas from a smelting reductionfurnace. Such off-gases normally contain carbon monoxide and somehydrogen. In addition hot off-gas from a smelting reduction furnacefrequently contains particles of metal oxide ore, particles of ore thathave been partially reduced and droplets of metal. The outlettemperature of such off-gases normally exceeds the sticky temperature.Consequently, when off-gases are cooled, care must be taken to avoidagglomeration of the particles and accretion thereof on apparatus.

Agglomeration and accretion of sticky particles can be avoided byemploying either or a combination of two techniques. The first techniqueinvolves rapidly heating the metal oxide particles to a temperature wellin excess of the sticky temperature range and after a short intervalrapidly cooling the particles to a temperature below the stickytemperature range. The second technique involves heating the metal oxideparticles to a somewhat lower temperature which nevertheless involvesheating some of the particles to a temperature in excess of the stickytemperature range and causing the particles to enter flow patterns suchthat inter-particle collisions of hot sticky particles and collisions ofhot sticky particles with apparatus are minimized. This ensures that themetallic oxide particles can be maintained for a longer period withinthe sticky temperature range. This can be accomplished by first reducingthe velocity of the stream as it flows from a first end of a verticalchamber towards a second end and subsequently increasing the velocity asthe stream approaches the second end. In this way incoming particles areinitially entrained or if present in the hot reducing gas remainentrained in the higher velocity entrance stream but as the stream slowsas it moves towards the second end some of the particles diverge fromthe stream and fall back towards the first end. Whether or not aparticle will remain entrained depends upon a number of factorsincluding its density, size, surface area, surface roughness and itsposition in the stream. Particles remaining entrained in the stream coolbelow the sticky temperature range before reaching the outlet in thesecond end and particles which fall towards the first end also coolbelow the sticky temperature range before becoming entrained again inthe higher velocity entrance stream of reducing gas.

This invention also provides a treatment chamber for pre-heating andpre-reducing metal oxides by the process according to the invention. Theinternal configuration of the chamber and the inlet ducts for reducinggas are chosen to promote pre-heating and pre-reduction while minimisingagglomeration and accretion.

The treatment chamber comprises a body portion, a first end, a secondend, an inlet in the first end and an outlet in the second end. The bodyand each end are defined by walls which taper from the body portiontowards the inlet and the outlet respectively. The body portion has across-sectional area that is many times larger than that of the inletand the outlet. Furthermore, the chamber is sufficiently elongated toensure that the flow pattern hereinbefore described can be set up whenthe chamber is vertically oriented.

This invention further provides an apparatus for the smelting of a metaloxide ore, which incorporates the treatment chamber defined above.

A surprising advantage of the present invention is that it permitsoff-gases to be quenched while ensuring that any sticky solids or otherentrained material are cooled to temperatures at which agglomeration andaccretion is much reduced or prevented altogether. However, if theoff-gases contain a relatively high proportion of sticky solids it maybe necessary to alter certain variables of the process such as the rateof injection of fresh solid particles.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings. FIG. 1 illustrates one embodiment of theinvention.

Off-gases from a molten bath reactor pass to a pre-heating andpre-reduction chamber. Fresh ore is also passed to this chamber and fromthere to a settling chamber and finally to the molten bath reactor.

FIG. 2 illustrates another embodiment of the invention which featurestwo pre-heating and pre-reduction chamber operating in series.

FIGS. 3(a) and 3(b) illustrate another embodiment of the treatmentchamber according to invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the invention will be described with particularreference to chromite ore but it can be applied to any other oxide ore.However, it has particular application to oxide ores which have a`sticking point` at a temperature within or close to the temperaturerange at which relatively rapid reduction takes place. Certain oxideores contain more than one metal. For example, chromite contains iron aswell as chromium. Pre-reduction of either or both metals is anadvantage. It is recognised that pre-reduction may not reduce all theoxides or oxide components.

The invention may be used in conjunction with any reactor which producesoff-gases at elevated temperature with reducing potential. It is ofparticular use in conjunction with a molten bath reactor, for example, amolten ferroalloy bath reactor used to reduce chromite ore. Whereoff-gas is used, the process according to the invention performs thefunction of reaction and/or heating of the particulate material and theoff-gases is itself quenched. Any sticky or molten substance entrainedin the off-gas may adhere to the fine particles. In many cases thepresence of a certain amount of this substance is quite acceptable.

The invention utilises some of the chemical energy and sensible heat,derived from a pyrometallurgical reactor, to pre-heat and pre-reducemetal oxide ores. In one embodiment these particles of metal oxide oremay be heated to a very high temperature to obtain enhanced rates ofreduction of the metal oxides. Even if the temperatures employed arelower than those at which stickiness of particles is significant in aparticularly preferred embodiment, the process according to theinvention provides an improvement over the prior art by reducing contactof the particles in the stream of gas with each other and also withinterior surfaces.

The particles are subsequently cooled rapidly below their stickingtemperature.

Generally speaking hot off-gases from a suitable source are introducedthrough an inlet duct or ducts located close to one end of an elongatedpre-heating and pre-reduction chamber. The duct or ducts are preferablyalso located axially or close to the axis of the chamber.

The cross-sectional area of the pre-heating and pre-reduction chambershould be substantially greater than the corresponding cross-sectionalarea of the ducts to minimise direct contact of the off-gases with thewalls of the chamber.

The oxide particles should preferably be introduced into the chamber ata point or points in close proximity to the points of entry of theoff-gases. The ore inlet or inlets should preferably be orientatedsubstantially in alignment with the direction of bulk flow of the offgases and to some extend towards the axis of the chamber. A degree ofswirl may also be provided to the gases transporting the oxideparticles.

The oxide particles may originate from bulk storage or from anotherchamber operated in conjunction with the first-mentioned chamber. Theoxide particles are transported to the chamber by any suitable gasincluding off-gas which has been completely oxidised.

The oxide particles are introduced into the off-gas stream in thetreatment chamber in such a manner that they are entrained in the bulkflow of gas, minimising contact between particles and with interiorsurfaces. The fine metal oxide particles are heated rapidly to very hightemperature through contact with the hot off-gases. It is surmised thatcertain flow patterns set up within the chamber near the wall of thechamber have a major role in the prevention of accretion andagglomeration. Heat is lost through the wall of the chamber. At thestage of discharge from the chamber, the particles in the flow patternshave dropped in temperature below the sticking temperature.

Contact between hot oxide particles may be minimised by varying therelative proportions of hot gas and metal oxide. Usually the quantity ofgas available is `given` and is dependent upon the operation of thefurnace, for example. Thus the feed rate of new metal oxide particles ismatched to the flow of gas. This factor should also be taken intoaccount in the design of the chamber, for example, to avoid unnecessarychoke-points and to promote gas flow, which after entry into thechamber, should be substantially axial.

The chamber may be fitted with a riser and in this case, the particleswill cool further before entering the means to conduct the particles tothe next treatment stage.

As mentioned previously, the internal configuration of the chamber andthe inlet ports is chosen to promote pre-heating and pre-reduction andto minimise agglomeration of particles and accretion on interiorsurfaces. The shape of the or each port or inlet duct is selected toprovide smooth, rounded interior surfaces with the minimum of dead spaceto reduce or eliminate agglomeration of particles from the molten bathreactor around the port or duct.

Once the particles have been reduced to the desired temperature they maybe withdrawn from the chamber and passed to further processing, forexample, in a cyclone.

A suitable temperature gradient may be achieved by means of externalcooling.

The residence time of the particles in the region of elevatedtemperature may be controlled by adjusting flow rates of one or both ofthe furnace off-gases and the carrier gas.

It should be noted that the oxide particles may be passed once throughthe chamber or more than once, as required.

In one embodiment of the invention a flux or fluxes may be added toeither the fresh oxide feed or the recycled oxide feed, the flux orfluxes being entrained in the entrainment gas.

It may be advantageous to inject some carbonaceous material andoxygen-containing gas which, when conbusted, may be used to boost thetemperature within the chamber to increase the rate of reduction.

Turning now to a more detailed consideration of the use of off-gasesfrom, for example, a molten bath reactor, which contain a relativelyhigh concentration of carbon monoxide and hydrogen, the exit temperatureof these gases may range from 1400° to 1800° C.

These off-gases are passed to a treatment chamber 1 as shown in FIG. 1,where they enter the chamber from inlet duct 2. Duct 2 is shown locatedupstream from ports 3 and 4 through which pass fresh ore in a carriergas. (Location upstream is the usual location.) The spatial relationshipof duct 2 to ports 3 and 4 and the internal dimensions of 1 are chosento promote rapid reduction of the ore particles in the general regionnear ports 3 and 4, followed by cooling as they penetrate further intothe chamber. In this way melting of the particles is minimised andstickiness reduced. The geometry of duct 2 and ports 3 and 4 and theirspatial relationship are chosen to reduce contact of the particles withthe wall of the chamber, reducing or preventing build-up on the wall.

As the stream of off-gases travels through the chamber, its velocitydecreases since the cross-sectional area of the chamber is many timesgreater than that of the inlet duct 2. However, as the off-gasesapproach the outlet, the velocity of the stream progressively increasesdue to the progressive reduction in cross-sectional area of the chambernear its outlet. As a result of these velocity variations, flow patternscan be established which reduce the propensity of the particles toagglomerate or to accrete to apparatus.

Gas and entrained solids pass to separator 5 from which waste gas passesto gas cleaning. Solids from 5, still entrained in gas, pass to streamdivider 6. All of the solids may be passed to a molten bath reactor 7 ora proportion may be returned to treatment chamber 1.

Turning now to FIG. 2, two treatment chambers 8 and 9 are provided inseries. Off-gases from molten bath reactor 10 pass to chamber 8 wherethey combine with entrained solids originating from separator, chamber 9and ultimately from a fresh feed source (not shown). Gas and solids fromchamber 8 are passed to separator 12, the separated gas being passed tochamber 9 and the solids to splitter 13. The solids from splitter 13 aredivided, one portion being passed to reactor 10 and the other portion tochamber 9. Ducts and ports in chambers 8 and 9 are similar to those inchamber 1 in the embodiment shown in FIG. 1.

In the embodiment shown in FIG. 2, the fresh solids are injected intothe cooler treatment chamber 9, before being passed via separator 11 tohotter chamber 8.

In both of the embodiments shown in FIGS. 1 and 2, the solids which arepassed to the respective molten bath reactor are conveniently admittedthrough the top of the reactor but may be injected at other points inthe reactor. At the top of the reactor the entry port may be separate orcombined with the entry port for oxygen-containing gas, for example,through an annulus surrounding the entry port for oxygen-containing gas.

In the embodiment of FIGS. 3(a) and (b), cooler particulate material isintroduced into chamber 14 near the chamber exit but in proximity to thechamber walls, so as to provide a falling curtain of cooler particlesadjacent to the interior surface. These particles, as they fall towardsthe off-gas entry duct of the chamber become entrained in the flow ofoff-gas, and leave the chamber through the gas and solids exit duct. Theparticles enter chamber 14 through internal, annular distributor 15. Thefalling curtain is designated 16 in FIG. 3(b), which represents across-sectional view along line A--A of FIG. 3(a). Duct 17 and ports 18and 19 are similar to those shown in FIG. 1.

Generally speaking, hot solids, which have a tendency to agglomerateduring prior processing, are not readily amenable to introduction intomolten baths via tuyeres or injectors. In consequence of their fineparticle size, the pre-heated solids produced according to thisinvention are well-suited to conveying and injecting into a molten bathprocess.

In both of the embodiments shown in FIGS. 1 and 2, the dotted arrow tothe left of the figure indicates injection of carbonaceous material andoxygen-containing gas. However, the region of injection must becarefully selected so as not to interfere with the mechanism of rapidheating of the particles following by cooling. The rate of reduction mayalso be boosted by injection of, for example, carbon monoxide, throughduct 2.

In the specification the term `oxygen-containing gas` refers to pureoxygen and gas containing oxygen, including air and oxygen-enriched air.

In the specification the term `carbonaceous material` refers to anycarbon-based material which can be burned to produce a suitably hightemperature and includes: anthracite, bituminous or sub-bituminous coal,coking or steaming coal, coke, lignite or brown coal, lignite or browncoal derived char, heavy petroleum residues and natural gas. The ligniteor brown coal may have been densified using the process disclosed inAustralian patents Nos. 561584 and 588565 and lapsed application No.52422/86. A process for preparing a char from such a densified productis disclosed in Australian patent application No. 52234/86.

In the specification the molten bath reactor from which the hotoff-gases are supplied may, for example, be any of the following: molteniron bath reactor, deep slag process reactor, ferroalloy bath reactor,non-ferrous bath reactor, or any other pyrometallurgical process bathreactor that discharges hot off-gases.

The method can be combined with any reduction or melt-down processes inwhich the prereduced ores are completely reduced. These may be eitherknown smelting reduction units or, for example, shaft furnaces, rotarykilns, fluidization or circulating fluidization.

A particularly advantageous application of the method, however, is tooperate it together with a smelting reduction vessel. The waste gasreleased from the smelting reduction reactor can be used directly in thepre-reduction chamber for reducing granular metal ores, and theresulting pre-reduced material completely reduced thereafter in thesmelting reduction vessel. This combined process offers a number ofbenefits. The impurities in the waste gas from this smelting reductionvessel whether dust, sticky particles or fine metal droplets, areacceptable in the reaction vessel and settle for the most part on theore particles. The high temperature of the waste gases from the smeltingreduction vessel provides the thermal energy necessary for heating thegranular metal ores, and the ore is pre-reduced by the high CO and H₂present in the waste gas. This involves a cooling effect, and thegranular metal ores additionally cool the waste gas to a desired,controllable degree. The stated features of the process allow, all inall, for a selective control of the degree of reduction of the ores, thetemperature and the flow conditions in the reaction vessel.

We claim:
 1. A process for pre-heating and pre-reducing a metal oxidecomprising the step of introducing metal oxide particles into a streamof hot reducing gas to heat to reduce, at least partially, the metaloxide particles wherein the hot reducing gas has a temperature in excessof that at which the metal oxide particles, or the metal oxide particleswhile contained in the stream of reducing gas exhibit stickycharacteristics.
 2. A process according to claim 1 further comprisingthe steps of:heating oxide particles to a temperature in excess of thatat which they exhibit sticky characteristics and thereafter quenchingsaid metal oxide particles to a temperature below that at which theyexhibit sticky characteristics.
 3. A process according to claim 1further comprising the steps of:forming the stream in a chamber andcausing the metal oxide particles introduced in the stream to enterdesired flow patterns in which interparticle contact is minimized andsticky particles are cooled below their sticky temperature beforecontacting internal walls of the chamber.
 4. A process according toclaim 3 further comprising the step of entraining the metal oxideparticles in a gas to form a stream of entrained metal oxide particlesto mix with said stream of hot reducing gas to form a stream of metaloxide particles and hot reducing gas.
 5. A process according to claim 3further comprising the steps of:progressively reducing the velocity ofthe stream of metal oxide particles and not reducing gas as the streamtravels through the chamber from a first end towards a second end of thechamber and then causing the velocity to increase as the second end isapproached, whereby said metal oxide particles are caused to enter thedesired flow patterns.
 6. A process according to claim 1, wherein thehot reducing gas is an off-gas from a smelting reduction furnace.
 7. Aprocess according to claim 6, wherein the temperature of the off-gas isin the range from 1400° C. to 1800° C.
 8. A process according to claim1, wherein the metal oxide particles are comprised of a chromite ore. 9.A process according to claim 4, wherein a degree of swirl is imparted tothe stream of metal oxide particles and hot reducing gas by the streamof entrained metal oxide particles.
 10. A process according to claim 1,wherein a portion of the metal oxide particles comprise recycledparticles of metal oxide.
 11. A process according to claim 4 furthercomprising the steps of:progressively reducing the velocity of thestream of metal oxide particles and not reducing gas as the streamtravels through the chamber from a first end towards a second end of thechamber and then causing the velocity to increase as the second end isapproached, whereby said particles are caused to enter the desired flowpatterns.